Blue summer skies finally reached Brussels this weekend, and with it came the fluffy aircraft contrails that crisscross Belgium’s busy skies. These white stripes are firmly in the EU’s sights due to their serious climate impact, distinct from and in addition to the warming from aircraft CO2 emissions.

As a result EU law requires that from 2025, airlines’ ‘non-CO2 emissions’ must be monitored, reported, and verified, for all flights departing from the EU. As well as contrails, monitoring efforts will cover nitrous oxides, sulfur dioxide, and soot emissions.

Airlines are pushing back against these requirements, saying the science is not yet strong enough to support an obligatory monitoring regime.

This is not the only scientific point that airlines and governments disagree on.

The EU’s position, backed by an extensive 2020 report, is that non-CO2 aircraft emissions have a climate impact at least or as great as the warming from aircraft’s CO2 emissions.

The International Air Transport Association (IATA), which represents both European and international airlines, accepts that non-CO2 emissions have a warming impact overall, but says there is much less scientific consensus on the magnitude of warming.

But monitoring these non-CO2 emissions is crucial, not only for policy making but also for day-to-day mitigation action.

One point on which all agree is that contrails’ climate impact stems from a small number of flights, in certain regions and at certain times. Aircraft flying high in the atmosphere through cool and humid air have the greatest impact – particularly if the contrails persist at night.

With strong data on when and where contrails are likely to form and persist, individual aircraft should be able to avoid in the future these zones, and seriously reduce their climate impact.

But this is as far as consensus goes. Airlines say that the science cannot yet identify the climate impact of non-CO2 emissions for individual flights. Premature operational maneuvers to avoid non-CO2 emissions could have an overall worse impact, according to this view. These maneuvers entail more fuel burn and therefore greater CO2 emissions, so if they fail to reduce contrails then the outcome is a net negative for the climate.

In an April letter seen by Euractiv, IATA asked the European Commission to apply mandatory monitoring only to flights within the EU. This is an old chestnut – a similar argument led to the Commission ‘temporarily’ exempting extra-EU flights from paying for the CO2 they emit. This exemption began in 2012 and is set to continue until at least 2027.

But while this tactic may have worked for the airlines in the past, it also risks creating divisions within their ranks.

Yesterday the NGO, Transport & Environment, wrote to the Commission a joint letter supporting mandatory monitoring of non-CO2 emissions. It called for extra-EU flights to remain in scope of monitoring efforts, and made practical proposals as to how these requirements could be pragmatically implemented.

Amongst the letter’s signatories was easyJet – a primarily intra-EU airline with zero interest in seeing its international competitors let off the hook.

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